1) You want to be able to watch the TV, without having the TV's sound on, instead listening to music.

2) You're currently using an "older" Yamaha receiver as a switch box for the TV and sound.

3) The TV is connected on the HDMI2 input on the Yamaha via an HDMI cable.

4) An Apple TV is connected to the Yamaha with an HDMI cable.

5) You have a Sonos CONNECT attached to the AV1 input on the Yamaha with an optical cable.

6) You have a 5.1 system of speakers connected to the Yamaha receiver.

So, what you need to do is separate the Yamaha as acting as the "switch" that controls what is on the TV and sound. Currently, it's acting as both sound and vision controller. But, if you were to separate the Apple TV, and plug it directly into the TV, which would then allow you to listen to anything you want, you'd lose your surround sound capabilities. Not ideal.

There's no effective way to do what you want, unless you instead get some other Sonos speakers. Because your Yamaha is driving all of your surround system, it has to be in the path of the video, and will control what is heard based on what is shown on the TV, and give you surround sound.

Your best bet would be to get an extra Sonos speaker, and use that to play whatever music you want while the TV is on, and muted.